1. Field of the Invention
This invention relates to a process for producing cellulose diacetate having improved properties. More specifically, this invention relates to a process for producing cellulose diacetate having reduced delustering tendency in hot water.
The cellulose diacetate in this invention denotes cellulose diacetate having a combined acetic acid amount of 54 to 57%.
2. Description of the Prior Art
When subjected to treatment with hot water, heated steam or the like in a dyeing process or the like, fibers from cellulose diacetate generally lose their characteristic luster and clarity and whiten. This phenomenon is referred to as the "delustering phenomenon".
The delustering phenomenon impairs not only the surface characteristics, such as gloss luster and dyeability, of cellulose acetate fibers, but also their physical characteristics (e.g., elongation and tenacity), and consequently reduces the merchandise value of the product. In particular, when synthetic fibers having higher thermal stability such as polyester fibers are used as blend-woven or blend-knitted fabrics with cellulose diacetate, it is frequently necessary to perform the hot water treatment, heated steam treatment, etc., of these fabrics at temperatures as high as about 120.degree. C. in the dyeing process. Accordingly, the delustering phenomenon of cellulose diacetate is an especially important problem in this technical field.
In the production of cellulose diacetate generally used in the art, the first step is to pre-treat cellulosic raw material such as wood pulp or cotton linters with a suitable amount of acetic acid and place the pre-treated material in a cooled acetylating mixture consisting of acetic acid, acetic anhydride and a catalyst to form primary cellulose acetate (completely esterified cellulose). The amount of acetic anhydride is in excess of the amount stoichiometrically required for reaction with the cellulose, the water in the cellulose, and the traces of water in the acetic acid, acetic anhydride and catalyst. Sulfuric acid is frequently used as a catalyst for commercial operations.
In the next step, an aqueous solution of a neutralizing agent (such as a carbonate, hydroxide, oxide or acetate of calcium, magnesium, iron, aluminum or zinc, preferably magnesium acetate) is added to the primary cellulose acetate to hydrolyze the acetic anhydride remaining after acetylation and to neutralize part of the catalyst used in the esterification. The resulting primary cellulose acetate is then hydrolyzed by maintaining it at 50.degree. to 90.degree. C. while a small amount of the acetylation catalyst (e.g., sulfuric acid) still remains, to thereby transform it to secondary cellulose acetate having a prescribed degree of acetylation and a prescribed degree of polymerization.
When secondary cellulose acetate having the prescribed degrees of acetylation and polymerization is obtained, the catalyst remaining in the ripened solution is completely neutralized with a neutralizing agent of the types exemplified above. Occasionally this neutralization is not performed. The hydrolyzed (ripened) solution is then poured into water or dilute acetic acid (or water or dilute acetic acid is poured into the hydrolyzed (ripened) solution) to separate the secondary cellulose acetate, which is washed and stabilized to obtain the final product. The time required for hydrolysis by this method, although it varies somewhat depending upon the composition and the temperature of the ripening bath, etc., is from several hours to 10 plus several hours which is comparatively long. Films from the cellulose acetate so obtained generally whiten and deluster when brought into contact with hot water at 80.degree. to 90.degree. C.
Several attempts have been made at reducing the delustering tendency of cellulose diacetate in hot water in the past. Attempts have been made to improve the cellulose diacetate produced by conventional method by an after-treatment. These after-treatments include (1) balancing acidic residue in the cellulose diacetate with an equivalent of a metal ion in a purifying step to increase the stability of the cellulose diacetate (Japanese Patent Publication No. 37203/72); (2) adding various metal salts in a washing step after-treatment (Japanese Patent Publication No. 10609/74); (3) adding a minute amount of an organic acid or its ester to a spinning dope containing cellulose diacetate (e.g., Japanese Patent Publication Nos. 3228/70, 11761/73 and 11762/73); (4) adding a minute amount of a nitrogen-containing organic compound to a spinning dope containing cellulose diacetate (Japanese Patent Application (OPI) No. 30753/73 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") and Japanese Patent Publication No. 10610/74); (5) removing a low degree of acetylation portion and a low degree of polymerization portion from the cellulose diacetate flakes by extraction (Japanese Patent Publication Nos. 23543/73 and 6229/75); and (6) treating acetate fibers with superheated steam (Japanese Patent Publication No. 3271/69). However, these methods do not offer a complete solution to the problem of reducing the delustering phenomenon because they complicate the process or their effects have poor reproducibility.
On the other hand, aside from the purpose of reducing delustering in hot water, it has been suggested to produce cellulose diacetate continuously in which the hydrolysis reaction is performed at relatively high temperatures to shorten the hydrolysis time. For example, Japanese Patent Application (OPI) No. 17580/73 discloses a process for continuously producing secondary cellulose esters from completely esterified cellulose starting material, characterized in that a completely esterified cellulose solution with the strongly acidic catalyst having been completely neutralized is maintained at a temperature of from about 125.degree. C. to 170.degree. C., and the completely esterified cellulose is hydrolyzed. The Publication states that this process leads to the improvement of the quality of acetate flakes such as the viscosity ratio, clarity, and filterability.
Japanese Patent Application (OPI) No. 17583/73 discloses that flakes having superior filterability and uniform quality can be obtained by mixing a neutralizing agent for a strong acidic catalyst, such as magnesium acetate solution, with the reaction solution at a time when completely esterified cellulose is formed, thereby to reduce the amount of the catalyst to 1.5 to 4 wt% based on the cellulose, and hydrolyzing the cellulose at 90.degree. to 120.degree. C. for 30 to 60 minutes.
British Pat. No. 620,726 discloses that cellulose diacetate having superior stability is obtained by adding a neutralizing agent in a nearly equivalent amount (the "nearly equivalent amount" is defined as the remaining of about 0.15% of the acid catalyst) based on the strongly acidic catalyst used for esterification of the reaction solution at a time when the completely esterified cellulose is formed, and hydrolyzing the esterified cellulose at a temperature of at least 125.degree. C.
It has now been found that the secondary cellulose acetate obtained by using these high-temperature hydrolysis methods is superior in its resistance to delustering in hot water in comparison to the cellulose acetate obtained by conventional methods in which hydrolysis is carried out at relatively low temperatures. The former product, however, is highly colored.